1. Field of the Invention
This invention relates to transmit/receive (Tx/Rx) antenna switches, and more particularly to frequency tunable Tx/Rx antenna switch using a tuned PIN diode RF switch topology.
2. Description of the Related Art
Transmit/Receive (Tx/Rx) antenna switches are a class of tuned Single-Pole Double-Throw (SPDT) switches used by designers of communications transceivers to alternately connect the transceiver's antenna to either the transmitter or to the receiver. The antenna switch should provide high isolation and low insertion loss. Isolation is a measure of the RF power through the switch that is not transferred to the load. In other words, isolation is a measure of how effectively the switch is turned OFF. Isolation is calculated as the difference between the power measured at the switch output port with the switched biased ON and the power measured at the switch output port with the switch biased OFF. Insertion loss is the transmission loss through the switch. Insertion loss absorbs signal power.
A PIN (p-type-intrinsic-n-type) diode is a diode with a wide, lightly doped ‘near’ intrinsic semiconductor region between is p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts. The wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (one typical function of a diode), but it makes the PIN diode suitable for attenuators, fast switches, photodetectors, and high voltage power electronics applications.
The use of PIN diodes as the switching element in the Tx/Rx antenna switch is based on the difference between the PIN diode reverse and forward bias characteristics. At lower RF frequencies, f<2 GHz, the PIN diode appears to be a very small impedance under forward bias and a very large impedance under reverse bias. Most switch designs use a difference in reflection, rather than dissipation, to obtain switch performance. Very little power is dissipated by the PIN diode itself, thus permitting small devices to control relatively large amounts of RF power. The basic PIN diode switch is broadband. There are many different switch topologies e.g. shunt, series, series-shunt that use PIN diodes. The various topologies are detailed in CH. 2 PIN Diode RF Switches of The PIN Diode Circuit Designer's Handbook, Microsemi Corporation, 1998, which is hereby incorporated by reference.
The electrical performance (high isolation, low insertion loss) can be enhanced by inserting quarter-wavelength transmission lines between the antenna port and the PIN diodes. When ON, the antenna port sees a low loss quarter-wave transmission line. When OFF, the antenna port see a high loss (open) circuit. However, the bandwidth is now constrained to less than an octave. These switches are referred to as Tuned Switches. See FIG. 2.4 in CH 2 as an example of a Tuned Shunt SPDT Switch.
The Tx/Rx antenna switch typically uses a Tuned SPDT Switch topology for the improved electrical performance. FIG. 2.11 in CH 2 illustrates an embodiment of a Tx/Rx Antenna Switch in a Series-Shunt topology that uses a single quarter-wavelength transmission line to protect the receiver. Forward biasing the PIN diodes connects the transmitter to the antenna. The low impedance of PIN diode D1 that terminates the quarter-wavelength transmission line protects the receiver. Reverse biasing the PIN diodes, isolates the transmitter by the high reactance of PIN diode D1 and the quarter-wavelength line connects the receiver to the antenna.
FIG. 4 of U.S. Pat. No. 5,909,641 illustrates another embodiment of a Tx/Rx Antenna Switch in a Shunt topology that uses a pair of quarter-wavelength transmission lines to protect both the transmitter and the receiver. Forward biasing PIN diode 45 into a low-impedance state connects the antenna port to the transmitter. This low impedance is transformed into a high impedance at the antenna port the through quarter-wavelength transmission line 43. When diode 44 is forward biased into a low-impedance state, the antenna port is connected to the receiver port.